KDJ
RT_STRING is value of resource type identifiers for string resource type, number(WORD 2 bytes only) or \0-terminated string. Equals to 6. You can find its and of other predefined types values in Win32\kernel32.js of mine Win32 Constants Library (you should use it effectively like AkelPad.Include("win32\\kernel32.js"), instead of manual definition if those constants or direct usage of *magic numbers*).

IDS_STRING is resource item identifier, number(WORD) or string(\0 termonated). You must specified value you need to load. Where to obtain it? From the author of module you want to load from. Since you want to load it from AkelPad's language modules, you should get it source(download from Download section here). You need file AkelFiles/Langs/Resources/resource.h there are all resource ID's used in AP and language modules.

I cannot say more. It is number Windows use to identify language. In C code is obtained by using macro MAKELANGID(p,s) ((s<<10)+p).
Where
p - primary language code, like:
LANG_ENGLISH (equals 9) for English,
LANG_RUSSIAN =25
LANG_UKRAINIAN =34
LANG_POLISH =21
etc(list is long - 1024 items)
s - secondary language code. For most of languages valid values for it are only(generic):

You should use SUBLANG_DEFAULT for them since it used almost everywhere in resources. MAKELANGID(LANG_RUSSIAN,SUBLANG_DEFAULT)= 1<<10+25= familiar for you 1049 = 0x0419
Some languages are a bit harder to encode, for instance:
LANG_ENGLISH can be:

I just realised I mislead you a little. If you going to seek localised version of button on standard AP's dialog, you must load dialog resource using resource type constant RT_DIALOG =5, not string resource, and in returned piece of memory you must parse it as DLGTEMPLATE or DLGTEMPLATEEX structure(see MSDN).

FeyFre and VladSh, Thank you for your criticism.
FeyFre
Zen-coding is a great toolkit for html and css workflow, and it has been renamed and moved to emmet for a while. It seems some update in it. Could you take a little time to update your script with that?

KDJ
First of all you Resource type argument is second, not third. I.e You should comment 16 line and uncomment 17.
Next, in order to determine error you should use GetLastError function immediately after call:

In script you posted here lasterror will be ERROR_RESOURCE_TYPE_NOT_FOUND=1813
(you can obtain human readable error text by calling function FormatMessageW/A)
if you fix 16,17 line you will get good result, or another error(you will get ERROR_RESOURCE_NAME_NOT_FOUND=1814).
And it is not their error. It is peculiarities of FindResourceEx function. It works with raw data stored in resources directory of image(and returns raw data). Don't worry, what I say next not every developer for windows knows, and even I know it accidentally. RT_STRING resources are stored in chunks of string(16 per chunk). That resource ID you use( 150) it threats as Chunk ID, and tries to find it. Fails. So you must convert you string id into chunk id, get raw string chunk, and extract required string manually. I never done that before, but here you can see example(in Russian)
Use ResourceHacker to inspect String resources(you will see strings are chunked)

FeyFre
Zen-coding is a great toolkit for html and css workflow, and it has been renamed and moved to emmet for a while. It seems some update in it. Could you take a little time to update your script with that?